The Yagi-Uda dipole array is a directional antenna consisting of a driven element (usually a dipole or folded dipole) and additional parasitic elements (usually a referred to as a reflector and one or more directors). Also disclosing additional directors is U.S. patent application Ser. No. 12/383080 entitled “Multi-Element Patch Antenna and Method” by Michael Josypenko, filed Mar. 13, 2009, which discloses a driven antenna element mounted on a circuit board with a ground plane formed on the opposite side. At least one parasitic antenna element may be mounted coaxial with and spaced apart from the driven antenna element at an offset distance.
To reduce the radiation to the back-side of the monopole and increase its gain, a reflector layer can be added at the side of the monopole that is opposite to the director side. A conducting ground plane can function as such reflector, but it has to be placed a quarter-wavelength under the monopole in order to produce the right reflection phase. This narrow-band solution also has the disadvantage of increasing the dimension of the antenna in the direction perpendicular to the monopole plane.
By putting a perfect metal conductor behind an antenna, a reflection will occur at −180 degrees phase difference, which leads to cancellation of the radiating waves. Placement of the sheet at one quarter wavelength alleviates this problem but requires a minimum thickness or spacing of λ/4. However, spacing the antenna at one quarter wavelength of the center frequency so that the reflected wave and the radiated wave constructively combine (along the boresight of the antenna) tends to consume excessive space. Moreover, surface currents or waves may develop in the metal sheet, leading to the propagation of interfering waves of radiation.
In the article entitled “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band,” IEEE Transactions on Microwave Theory and Techniques,” Vol. 47, No. 11, November 1999, pages 2069-2074, herein incorporated by reference, there is described a type of metallic electromagnetic structure that is characterized by having high surface impedance, and although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, the surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The uses include low profile antennas.
The publication by E. Yablonovitch, entitled “Photonic band-gap structure,” J. Opt. Soc. Amer. B, Opt. Phys., vol. 10, pp 283-295, (February 1993) describes how a photonic semiconductor can be doped, producing tiny electromagnetic cavities. The article postulates that structures made of positive dielectric-constant materials, such as glasses and insulators, can be arrayed into a three-dimensionally periodic dielectric structure, making a photonic band gap possible, employing a purely real, reactive, dielectric response. The photonic band gap described in the Yablonovitch reference refers to the band gap or an area where electron-hole recombination into photons is inhibited.
Electromagnetic reflective structures are usually periodic consisting of metal patches that are separated by a small gap and vias or pins that connect the patches to the ground plane. The electrical equivalent circuit consists of a resonant tank circuit, whose capacitance is represented by the gap between the patches and the inductance represented by the via. See in this regard D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous, and E. Yablonovitch, “High-impedance frequency selective surface with forbidden frequency band,” IEEE Trans. Microwave Theory Tech., vol. 47, pp 2059-2074, November 1999, and/or D. Sievenpiper, “High-impedance Electromagnetic Surfaces,” Ph. D. dissertation, Dep. Elect. Eng. Univ. California at Los Angeles, Los Angeles, Calif., (1999), both of which are hereby incorporated by reference.
The electromagnetic reflective structures are in effect a magnetic surface at the frequency of resonance and thus have very high surface impedance. This makes a tangential current element close to the electronic band gap structure equivalent to two current elements oriented in the same direction without the electronic reflective structure, which helps to enhance the forward radiation instead of completely canceling it, as suggested by the image theory. This makes electronic reflective structures useful when mounting an antenna close to a ground plane, provided the antenna's currents are parallel to the electronic reflective structure. Electronic reflective structures have previously been known to operate over a very narrow band, and thus not useful with a broadband antenna.